A transferred impedance filter (TIF), also known as a transferred impedance filter (TIF), utilising N-path filtering techniques, such as disclosed in U.S. Pat. No. 7,187,230 and in “Tunable high-Q N-path band-pass filters: modeling and verification”, A. Ghaffari et al, IEEE J. of Solid-State Circuits, May 2011, can provide narrowband bandpass filtering centred at a switching frequency, that is, a local oscillator frequency. In US 2010/0097258 and in “A 900-MHz direct delta-sigma receiver in 65-nm CMOS”, K. Koli et al, IEEE J. of Solid-State Circuits, December 2010, a direct delta-sigma receiver is disclosed employing TIFs.
A known 4-path TIF mixes a signal to a desired bandpass frequency range from odd harmonics of a local oscillator (LO) signal, using a switching duty cycle of 25%. The TIF acts as a high impedance for input frequencies close to the switching frequency, forming a high-Q bandpass filter centred at the switching frequency. A basic TIF circuit is a capacitively loaded passive mixer, and therefore it will mix signals from all odd harmonics of the LO signal. The stopband attenuation of the circuit is limited by the on-resistances of the switches employed in the TIF, and, near odd harmonics of the clock signal, additional bandpass resonances occur. The transfer function of such a 4-path TIF has repeated, but slightly attenuated, bandpass responses on every odd harmonic of the LO signal. The use of a switching duty cycle shorter than 25% can slightly improve the bandpass frequency selectivity, although attenuation at the odd harmonics of the LO signal is severely degraded, as described by Ghaffari et al.
In order to improve the harmonic rejection capabilities, more phases than four can be used to construct 6 or 8-path TIFs, as disclosed by Ghaffri et al. For example, mixing down from odd harmonics of the LO signal can be reduced by using, for example, an 8-path TIF with a 12.5% switching duty cycle. That, however, leads to reduced attenuation at harmonics of the clock signal, rendering the benefits of the circuit rather limited, while significantly increasing the maximum VCO frequency required for generating the LO signal. The main reason for the reduced attenuation is the shorter switch on-time, which limits the filtering at odd harmonics of the LO signal, while slightly reducing losses in the desired passband. The TIF is usually connected in parallel with a passive LC-resonator, which reduces the losses in the TIF passband, although this is insignificant compared with the losses of the passive LC-resonator. Furthermore, the harmonic rejection capabilities of the LC-resonator alone are not enough for typical applications, for example for wireless local area network (WLAN) coexistence in cellular transceivers.
Therefore, there is a requirement for improved filtering.